Any signed experiment can be countersigned by a notebook member other than the experiment creator.
If you can countersign an experiment, the “Countersign” button will be enabled in the experiment navbar.

A click on this button will open a form asking for your password. This is a security measure required to make sure you actually are the owner of your account.

Type your password and click on the “Countersign” button.
This will mark the experiment as countersigned by you.